For years, projection systems have been used to project motion pictures and still photographs onto display screens. More recently, presentations using multimedia projection systems have become popular for purposes such as sales demonstrations, business meetings, and classroom sessions. In a common mode of operation, multimedia projection systems receive analog video signals from a personal computer (PC). The video signals represent still, partial-, or full-motion display images of the type rendered by the PC. The analog video signals are converted into digital video signals to control a digitally-driven light valve, such as a liquid crystal light valve (LCD), which form display images.
A popular type of multimedia projection system is a projector that incorporates a light source and optical path components upstream and downstream of the light valve to project the display images onto a display screen. Examples such projectors include those sold under the trademark LITEPRO.RTM. by In Focus Systems, Inc. of Wilsonville, Oreg., the assignee of the present application.
In LCD-based projectors, light from a light source is polarized by a back polarizer to obtain light with a selected pass orientation. The polarized light is then modulated by an informational pattern of active and inactive (blanked) pixels on a transmissive LCD. The active pixels rotate the polarized light by 90.degree., and the inactive pixels pass the polarized light without rotation. The modulated light then passes through a front polarizer (analyzer), which blocks the rotated light of the activated pixels, and passes the nonrotated light of the blanked pixels. The modulated light is then projected through a projection lens onto a screen for viewing.
A major design challenge for such LCD-based projectors is the substantial light loss due at the back polarizer. For example, simple polarization wastes about one-half of available light. To minimize this loss, polarization conversion devices have been developed to convert otherwise wasted light to the desired pass orientation. One such conventional device is a polarization conversion prism.
While conventional polarization conversion prisms work adequately, they are often fairly complex, being constructed of numerous pieces of glass defining complex optical paths. For example, U.S. Pat. No. 5,381,278 to Shingaki et al. discloses several conversion prisms having at least three prism components, and a bonded 1/4-wave plate and reflection plate. Light that is otherwise wasted is converted to the required pass orientation by passing through the 1/4-wave plate, being reflected off the reflection plate and passing back through the 1/4-wave plate for a total 90.degree. rotation. While such a prism works adequately, some light is lost in the reflection off the reflection plate and the double transmission through the 1/4-wave plate. This light loss tends to be increased by the converted light passing twice through a triangle prism component of the conversion prism. Moreover, the three-prism construction and the bonded 1/4-wave and reflection plates tend to be expensive to manufacture.
Conventional projectors also often have dissatisfactory brightness uniformity. Some projectors incorporate fly-eye lenses to integrate the light to improve the uniformity of projected images. Unfortunately, fly-eye lenses are becoming less applicable to today's multimedia projector designs, which require low cost, and toughness for portability. For instance, fly-eye lenses tend to be expensive and fragile. This drawback is worsened in that two fly-eye lenses are often required to produce the desired integration and imaging. Fly eye lenses also tend to make polarization conversion problematic and expensive, in that the rows of lenses making up a fly-eye lens may require a complicated polarization conversion prism with rows of prism components registered with the rows of lenses on the fly eye lens. Furthermore, fly-eye lenses tend to be relatively large in order to provide adequate light integration.
Conventional projectors also have a disadvantage in projecting an image with excess yellow color, resulting in the projected image having an undesirable green or orange tint. This coloration is due to the color spectrum of metal-halide lamps having a spike in the yellow region at about 577 nm.
Thus, in light of these disadvantages, it is a principle object of the present invention to provide a projector with inexpensive, compact and tough means to recover polarized light that would otherwise be wasted.
It is another object of the present invention to provide a projector optical system that eliminates unwanted yellow light from the image.
In accordance with a preferred embodiment of the present invention, a multimedia projector includes, from upstream to downstream, a light source, a light pipe having an inlet and outlet and constructed to receive light from the light source and to produce a unique projection display format as light is transmitted therethrough, a positive lens group including at least one positive lens, and a light valve with an active display region corresponding to the projection display format of the light pipe. The positive lens group images the outlet of the light pipe onto the light valve.
In accordance with another preferred embodiment of the present invention, a polarization conversion prism is positioned downstream of the light pipe.